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Transcript
Vol.2, No.10, 1066-1072 (2010)
Natural Science
http://dx.doi.org/10.4236/ns.2010.210132
Adsorption studies of cyanide onto activated carbon
and γ-alumina impregnated with cooper ions
Liliana Giraldo1, Juan Carlos Moreno-Piraján2
1
Facultad de Ciencias, Departamento de Química, Universidad Nacional de Colombia, Bogotá, Colombia; [email protected];
Facultad de Ciencias, Departamento de Química, Grupo de Investigación en Sólidos Porosos y Calorimetría, Universidad de los
Andes, Bogotá, Colombia; [email protected]
2
Received 5 May 2010; revised 17 June 2010; accepted 20 June 2010.
ABSTRACT
In this research, adsorption of cyanide onto catalyst synthesized with activated carbon and γalumina used supported and cooper has been
studied by means of batch technique. Percentage adsorption was determined for this catalyst
in function of pH, adsorbate concentration and
temperature. Adsorption data has been interpreted in terms of Freundlich and Langmuir equations. Thermodynamics parameters for the adsorption system have been determined at three
different temperatures.
Keywords: Activated carbon; Alumina; Cyanide;
Isotherms; Thermodynamic
1. INTRODUCTION
Waste water discharged by industrial activities is often
contaminated by a variety of toxic or otherwise harmful
substances which have negative effects on the water environment. For example, of metal finishing industry and
electroplating units is one of the major sources of heavy
metals such as (Zn, Cu, Cr, Pb etc.) and cyanide pollutants which contribute greatly to the pollution load of the
receiving water bodies and therefore increase the environmental risk [1-4]. Cyanide present in effluent water of
several industries. Cyanidation has dominated the gold
mining industry. In view of the toxicity of cyanide, and
the fact that cyanide is fatal in small dosages, authorities
have been forced to tighten up plant discharge regulations.
It is therefore vital to recover as much cyanide as possible, not only to meet standard requirements, but to strive
towards obtaining lower levels of free cyanide (CN-) in
tailing and plant effluent [4-8]. The solubility of gold in
cyanide solution was recognized as early as 1783 by
Scheel (Sweden) and was studied in the 1840s and 1850s
by Elkington and Bagration (Russia), Elsner (German)
and Faraday (England) [9]. Elkington also had a patent
Copyright © 2010 SciRes.
for the use of potassium cyanide solutions for electroplating of gold and silver [9]. Cyanide is a singly-charged
anion containing unimolar amounts of carbon and nitrogen atoms triply-bounded together. It is a strong ligand,
capable of complexing at low concentrations with virtually any heavy metal. Because the health and survival of
plants and animals are dependent on the transport of
these heavy metals through their tissues, cyanide is very
toxic. Several systems have been adopted for the reduction of cyanide in mill discharges. There are SO2 assisted
oxidation, natural degradation, acidification volatilization-reneutralization, oxidation and biological treatment.
However, in the first three processes, cyanide reduction
does not appear to meet the strict regulatory requirements,
and as for the fourth process, it is limited to certain climate conditions. The next best process used, is the oxidation with hydrogen peroxide where the cyanide concentration is reduced to low enough levels, but this process requires an expensive reagent which cannot be reused [10-16]. Activated carbon was used for the removal
of free cyanide from solution, but observed that copperimpregnated carbon yielded far better cyanide removal
[17]. However, did not test other metal impregnated carbons or different metal loadings on the carbon. The use
of a metal impregnated carbon system would therefore
be more effective in reducing cyanide concentrations in
solution. Due to the problem mentioned above, the study
on using activated carbon in the removal of free cyanide
is being done in our laboratory [18,19]. The adsorption
onto activated carbon has found increasing application in
the treatment of wastewater, as well as for the recovery
of metals from cyanide leached pulps. Activated carbon
has a great potential for cyanide waste treatment both in
gold extraction plants and effluent from metal finishing
plants and hence, it forms a subject studied in the present
work. Characterization of activated carbon shows that
surface area has an effect although the reactivity of the
surface as a result of oxygenated functional groups, e.g.
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L. Giraldo et al. / Natural Science 2 (2010) 1066-1072
carboxylate and phenolate is thought to be significant in
the sorption of metal cations. Pore size distribution has
been used to describe the internal structures and adsorption capacities of activated carbons [20]. The highly active surface properties of the activated carbon are attributed to the chemical functional groups and the internal
surface areas, which typically range from 500 to 3000
m2/g [21]. The effect of copper was studied in the adsorption of cyanide onto activated carbon. It was found
that the removal capacity was highly improved by the
presence of copper [18-21]. It is the aim of this research
to use of activated carbon (obtained from cassava peel)
[22,23] and alumina impregnated with cooper for obtain
catalyst for the removal of cyanide for dilute solutions.
The pertinent parameters that influence adsorption such
as initial cyanide (CN-) concentration, agitation time, pH
and temperature were investigated. Adsorption isotherms
at three different temperatures (i.e. 283 K, 313 K, 323 K)
have been studied. The adsorption data have been interpreted using Freundlich and Langmuir isotherms. Various thermodynamic parameters including the mean energy of adsorption have been calculated.
2. EXPERIMENTAL
All reagents used in the experimental work were of
analytical grade (E.MERCK)® Argentmetric (largely
AgNO3) titrations were employed for CN- determination
[8]. Stock solution of cyanide (1000 mg.L-1) was prepared by dissolving Sodium cyanide in distilled water.
The concentration range of cyanide prepared from stock
solution varied between 10 to 80 mg.L-1.
2.1. Preparation of Catalysts
The activated carbon used in this study was prepared
for by pyrolysis of cassava peel in presence of chloride
zinc (chemical activities) by our research group. Cassava
peel from Colombian Cassava cultives were impregnated
with aqueous solutions of ZnCl2 following a variant of
the incipient wetness method [22,23] with a specific surface area of 1567 m2.g-1.
One of them was obtained commercial (Sasol™) sample of γ-alúmina.
2.1.1. Impregnation
Catalysts (activated carbon with cooper was labeled
Cu-AC and γ-alúmina with cooper was labeled Cu-A)
were formulated using a solution of Cu(NO3)2.3H2O (5
wt.(%) Cu) as a precursor of the active agent because of
its water solubility, lower cost, and lack of poisonous
elements. Activated carbons from cassava peel and γ-alúmina were used as supports. There are two well known
methods of loading the metal precursor on the support:
incipient wetness impregnation and soaking method. In
Copyright © 2010 SciRes.
1067
106
this case, the second one was used; the activated carbon
and γ-alúmina were soaked in a copper nitrate solution
for 8 days under agitation until the equilibrium was reached. The ratio solid/solution (weight base) was equal to
6. The impregnated solids were filtered and scurried
during 24 hours. Then, they were dried at 378 K for 24
hours 12.
2.1.2. Thermal Treatment
The objective of this step is to transform the copper
salt (cupric nitrate) in copper oxides, which are the active catalytic agents. The high temperature used descomposes the nitrate releasing nitrogen oxides. The reactor
must work in a nitrogen atmosphere to avoid the combustion of the support. The catalysts were treated in the
activation reactor, at 830 K for 24 hours. After that, a
nitrogen flow was maintained until the reactor reached
room temperature 12.
2.2. Characterization
Nitrogen adsorption-desorption isotherms were performed using an Autosorb-3B (Quantachrome) equipment. Samples of 0.100 g were oven-dried at 378 K
during 24 hours and outgassed at 473 K under vacuum
for 10 hours. The final pressure was less than 10–4 mbar.
Textural parameters were derived from adsorption data.
The specific BET surface area was estimated 15. The
specific total pore volume (VT) was determined from the
adsorption isotherm at the relative pressure of 0.99,
converted to liquid volume assuming a nitrogen density
of 0.808 g.cm–3. The specific micropore volume (VDR)
was determined using the Dubinin-Radushkevich mode
l16. The pore size distribution (PSD) was analyzed using
the BJH method [17,21-24].
Acidity and basicity determinations of the support were
made by titrating the acid sites with a strong basic solution and the basic sites with a strong acid solution, following the protocol detailed by Giraldo et al. [22,23].
The point of charge zero (PZC) of the catalyst were
determined for a procedure very similar [24], which are
described here. In a beaker were added 0.1 g of finely
ground catalyst in an agate mortar and 20 mL of a 0.01
M KCl-0.004 M KOH. The solution was kept under constant stirring for 48 h. Then, titration was performed
with a 0.1 M HCl solution using a burette and a stream
of nitrogen to prevent carbon dioxide from the air is absorbed into the solution and the formation of CO3-2 and
HCO3-1. The titrant solution was added slowly 0.1 mL
and was recorded by adding the aggregate volume and
pH of the solution. Furthermore, the evaluation was carried 0.01 M solution of KCl-0004 KOH under the same
conditions but without catalyst. The PCC of the catalyst
was determined by plotting the pH of the solution against the volume of titrant solution to the solution without
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L. Giraldo et al. / Natural Science 2 (2010) 1066-1072
catalyst and the catalyst solution, the pH where these two
curves intersect corresponds to the PZC. Another way
interpret experimental data is to calculate the burden of
surface of the catalyst using the equations reported in
literature [24], in this case the pH at which the burden of
surface is zero corresponds to the PZC.
Studies of X ray diffraction (XRD) patterns were recorded at room temperature using a Rigaku diffractometer operated at 30 kV and 20 mA, employing Ni-filtered
Cu Kα radiation (λ = 0.15418 nm). The crystalline
phases were identified employing standard spectra software.
lume compared to that of the corresponding support. This
is mainly due to a pore blocking effect, especially micropores, which is evident from the analysis of the uptake
at low relative pressures in the adsorption-desorption isotherms for the catalysts compared to the supports. The
micropore volume and surface area using activated carbon is higher than that obtained from γ-alumina, as the
total pore volume for both materials.
The isotherms corresponding to both catalysts present
0.0029
2.3. Adsorption Studies
3. RESULTS AND DISCUSSIONS
3.1. Characterization
Nitrogen adsorption-desorption isotherms for the supports and catalysts are shown in Figures 1 and 2. The
textural parameters modeled from the adsorption data
are summarized in Table 1.
Both catalysts present lower surface area and pore voCopyright © 2010 SciRes.
0.0026
0.0023
Pore Volume [cc/μg]
0.0020
0.0017
0.0014
0.0012
0.0009
0.0006
0.0003
0.0000
0.000 6.247 12.494 18.741 24.987 31.234 37.481 43.728 49.975 56.222 62.469
Pore Width [Å]
Figure 1. Pore distribution for Cu-AC.
0.0091
DFT Differential Pore Volume Distribution
0.0082
0.0073
0.0064
Pore Volume [cc/μg]
The adsorption of CN- on activated carbon and γ-Alumina impregnated with Cu was studied by batch-technique [9]. The general method used for these studies is
described below: A known weight (i.e., 0.5 g of the CuAC or Cu-A) was equilibrated with 25 cm3 of the spiked
cyanide solution of known concentrations in Pyrex glass
flasks at a fixed temperature in a thermostated shaker
water bath for a known period of time (i.e. 30 minutes).
After equilibrium the suspension was centrifuged in a
stoppered tube for 5 minutes at 4500 rpm, was then filtered through Whatman 41 filter paper. All adsorption
experiments except where the pH was varied were done
at pH 7.20, which was obtained naturally at solution to
adsorbent ratio of 50:1. To study the effect of pH, in one
set of experiments the pH of the suspensions was adjusted by using NaOH/NH4OH and HNO3. The pH of solutions was in the range of 3.0-12. The amount of cyanide
adsorbed, “X” and the equilibrium cyanide concentration
in the solution, “Ce” was always determined volumetrically with standard silver Nitrate solution. Adsorption of
cyanide on Cu-AC and Cu-A was determined in terms of
percentage extraction. Amount adsorbed per unit weight
of the Cu-AC or Cu-A, X/m was calculated from the initial and final concentration of the solution, Adsorption
capacity for the adsorption of cyanide species has been
evaluated from the Freundlich and Langmuir adsorption
isotherms were studied at three different temperatures
(i.e. 283 K, 313 K, 323 K). The cyanide concentration
studied was in the range of 10 pmm to 80 ppm for 50:1
solution to the catalyst.
DFT Differential Pore Volume Distribution
0.0055
0.0046
0.0036
0.0027
0.0018
0.0009
0.0000
0.000 5.709 11.418 17.127 22. 836 28.545 34.255 39.964 45.673 51.382 57.091
Pore Width [Å]
Figure 2. Pore distribution for Cu-A.
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L. Giraldo et al. / Natural Science 2 (2010) 1066-1072
Table 1. Textural parameters of catalyst.
Sample
BET surface area (m2.g–1)
Pore volume (VT)
(cm3.g–1)
Specific micropore volume (VDR) (cm3.g–1)
AC
1567
0.6641
0.6245
Cu-AC
1245
0.5478
0.6042
γ-Alúmina
245
0.2454
-
Cu-A
207
0.2087
-
a hysteresis loop. The uptake at high pressure rises significantly for Cu-A, which indicates a considerable presence of mesoporous. The BJH pore size distribution (Figures 1 and 2) shows important mesoporosity in the range
of 40-90 Å for both catalysts, especially in Cu-AC, in
agreement with its hysteresis loop.
Figures 3 and 4 show the curves obtained in the point
of zero charge determinations using the mass titration
method. All these results, pH, acidity, basicity and pzc,
0,05
surface charge,(C/m2)
0,03
0,01
6
7
8
9
10
11
12
13
14
-0,01
-0,03
-0,05
demonstrate that the surface of both supports are strongly basic (Table 2).
The X Ray diffraction patterns for both catalysts, CuAC and Cu-A respectively are shown in Figures 5 and 6.
The profiles obtained show that the main oxidized metal
compound corresponds to CuO. The signals of Cu2O and
Cu are also present in the spectra, specially the last one.
The presence of CuO was expected because the metal
appears as divalent ion in the precursor salt. The reduced
species (Cu+ and CuO) could be produced by the reducing action of the carbon surface during the thermal treatment at high temperatures.
The adsorption of cyanide on the catalyst Cu-AC and
Cu-A were studied as a function of shaking time in water
Bath shaker (Labconco 3535 US), pH, adsorbate concentration and temperature for known cyanide concentration
at 313 K. The results are interpreted in terms of percentage adsorption. The variation of % adsorption with different intervals of time ranging from 2 minutes to 48
hours is illustrated by Figure 7 which shows that the
adsorption of cyanide at 25 ppm as well as 50 ppm concentration on Cu-AC and Cu-A is rapid at 313 K and
equilibrium reached instantaneously after mixing cyaTable 2. Physicochemical characteristics of the catalyst.
pH
Figure 3. PZC for Cu-A catalyst.
Basicity (mmol.g–1)
pHPZC
8.6
Acidity
(mmol.g–1)
0.27
0.81
9.0
5.8
0.20
0.45
6.0
Support
pH
Cu-AC
Cu-A
0,04
30000
0,02
0
2
4
6
8
-0,02
-0,04
12
14
20000
15000
10000
5000
-0,06
0
-0,08
0
pH
Figure 4. PZC for Cu-AC catalyst.
Copyright © 2010 SciRes.
10
Intensity, u.a.
surface charge,(C/m2)
25000
0
10
20
30
40
50
60
70
80
90
2 Theta
Figure 5. DRX for Cu-AC catalys.
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L. Giraldo et al. / Natural Science 2 (2010) 1066-1072
Table 3. Dependence of absorbance concentration relative to
CN- on Cu-A catalyst at 293 K.
8000
7000
Amount of CN- Amount of CN- in Amount of CNAdsorption
in taken
sol. At equilibrium Adsorbed
pH
6000
2,06
4,57
6,54
7,28
9,14
11,34
12,75
Intensity,u.a.
5000
4000
3000
2000
(ppm)
(ppm)
(ppm)
(%)
20
20
20
20
20
20
20
6,48
5,44
4,11
2,13
4,11
6,55
7,76
13,52
14,56
15,89
17,87
15,89
13,45
12,24
67,60
72,80
79,45
89,35
79,45
67,45
61,20
Table 4. Dependence of absorbance concentration relative to
CN- on Cu-AC catalyst at 293 K.
1000
0
0
10
20
30
40
50
60
70
80
90
Figure 6. DRX Cu-A catalyst.
103
98
%age Adsorption
93
88
Cu-A25
83
Cu-AC50
78
Cu-A50
73
Cu-AC25
68
63
58
0
15
30
45
60
75
90
105 120 135 150
Time,minutes
Figure 7. Effect of shaking time % age Adsorption at 25 ppm
and 50 ppm for the synthesized catalyst.
nide solution with the catalyst. However, an equilibriumis reached faster with the catalyst of Cu-AC than with
Cu-A; that is associated with the greatest amount of
copper that is achieved on the activated carbon adsorb
taking into account their surface properties and specific
chemical the PZC allowing CN ions adsorb more rapidly
on the catalyst surface Cu-AC on the Cu-A.
No significant change in % adsorption values was observed up to 48 hours, which indicates that surface precipitation as well as ion exchange may be the possible
adsorption mechanism. Therefore, equilibrium time of
20 minutes was selected for all further studies. The adsorption is pH dependent, a much greater adsorptive ca
pacity for cyanide was observed in neutral solution for
both catalyst; moreover it is more higher adsorption capacity for Cu-AC, i.e., pH 7- 8.0 (Table 3 and Table 4).
Because when the pH is reduced, surface charge of the
particles becomes increasingly positive and because of
the competition of the hydrogen ions for the binding sites,
Copyright © 2010 SciRes.
Amount of Amount of CN- in Amount of CNAdsorption
CN- in taken sol. At equilibrium Adsorbed
pH
2 Theta
2,06
4,57
6,54
7,28
9,14
11,34
12,75
(ppm)
20
20
20
20
20
20
20
(ppm)
5,35
3,34
1,23
0,99
2,02
4,66
5,55
(ppm)
14,65
16,66
18,77
19,01
17,98
15,34
14,45
(%)
73,25
83,30
93,85
95,05
89,90
75,70
72,25
metal ions tend to desorbs at low pH region, as well a
small decrease in cyanide adsorption was observed at pH
higher than 9.0. This behavior may be due to the formation of soluble cyanide complexes, which remain in solution as dissolved component. Similarly adsorption of
cyanide as a function of its concentration was studied by
varying the metal concentration from 10ppm to 80 pmm,
% age adsorption values decreases with increasing metal
concentration (Table 5), which suggest that at least two
types of phenomena (i.e. adsorption as well ion-exchange)
taking place in the range of metal concentration studied,
in addition less favorable lattice positions or exchange
sites become involved with increasing metal concentration.
The adsorption in aqueous solutions and adsorption
isotherms at three different temperatures (i.e. 278 K, 298
K, 323 K) were obtained by plotting the amount of cyaTable 5. Dependence of adsorbate concentration relative to
CN- on Cu-AC (this catalyst present major adsorption).
Amount of
Adsorbent
CN- in taken
Amount of
CN- taken
(mg
(ppm)
(ppm)
(ppm)
(%)
500
500
500
500
500
500
500
5,00
10,00
20,00
40,00
60,00
80,00
100,00
2,42
3,12
1,41
15,65
37,98
41,09
54,58
2,58
6,88
18,52
30,35
38,44
38,91
45,42
51,60
68,00
92,60
75,88
64,06
48,63
45,42
Amount of
Amount of
Adsorption
CN- in soln.
CN- Adsorbed
at Equilibrium
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L. Giraldo et al. / Natural Science 2 (2010) 1066-1072
nide adsorbed on Cu-A and Cu-AC (mg/g) against metal
at equilibrium concentration “Ce” (mg/l). Adsorption of
cyanide decreases with increasing temperature. Two models, Langmuir and Freundlich equations, were used to
describe experimental data for adsorption isotherms.
The linear form of the Freundlich isotherm model is
given by the following relation:
(1)
where x / m is the amount adsorbed at equilibrium (mg/g),
Ce is the equilibrium concentration of the adsorbate
(mg/l), and KF and 1 / n are the Freundlich constants
related to adsorption capacity and adsorption intensity
respectively, of the sorbent. The values of KF and 1/n can
be obtained from the intercept and slope respectively, of
the linear plot of experimental data of log X / m versus
logCe. The linear form of the Langmuir isotherm model
can be represented by the following relation:
Ce / x / m = 1 / KLVm + Ce / Vm
Langmuir
Qo
ΔG° = - RTlnK
(3)
ΔG° = ΔH° -
(4)
K = - ΔH° / RT + Constant
(5)
R
Kf
n
Cu-A
317,2252 0,041956 0,99700 22,77397 1,759936 0,98713
Cu-Ac
297,2679 0,164927 0,99646 58,15241 2,573350 0,97419
Table 7. Values of thermodynamic data for Adsorption of CNon Cu-AC.
Temperature
K
ΔH°
kJ/mol
ΔG°
kJ/mol
ΔS°
J/K.mol
278
6.234
-8.856
0.0543
298
6.234
-6.636
0.0432
323
6.234
-12.666
0.0585
Table 8. Values of thermodynamic data for Adsorption of CNon Cu-A.
Temperature
K
ΔH°
kJ/mol
ΔG°
kJ/mol
ΔS°
J/K.mol
278
4.897
-6.863
0.0423
298
4.897
-6.963
0.0398
323
4.897
-10.183
0.0467
300
250
200
Alumina
Activated Carbon
Cu-A
Cu-AC
150
Langmuir
Freundlich
100
and
Tables 7 and 8 show the values of thermodynamics
parameters ΔH°, ΔS°, ΔG° for Cu-AC and Cu-A catalyst
synthesized. The positive value of ΔH° = 6.234 kJ/mole
for Cu-AC and 4.897 kJ/mole, which is calculated from
Eq.5 and Figure 5, confirms the endothermic nature of
the overall adsorption process. The positive value of ΔS°
suggests increased randomness at the solid/solution interface with some structural change in the adsorbate and
adsorbent and also affinity of the Cu-AC and Cu-A towards CN-; the values more highest of entropy indicate
spontaneous process. A negative value of ΔG° indicates
Copyright © 2010 SciRes.
R2
Alumina 221,9725 0,018648 0,99835 7,431714 1,477998 0,99653
(2)
where Vm and KL are the Langmuir constants related to
the maximum adsorption capacity and the energy of adsorption, respectively. These constants can be evaluated
from the intercept and slope of the linear plot of experimental data of Ce / X / m versus Ce. The Freundlich and
Langmuir adsorption isotherms are shown in Figure 3
and Figure 4 (the isotherms linearizated not shown here).
The related parameters of Langmuir and Freundlich models are summarized in Table 6. The results reveal that
both the Langmuir isotherm model adequately describes
better the adsorption data (See Figure 8).
Calculations of thermodynamic parameters:
Thermodynamic parameters such as Gibbs free energy
ΔG° (kJ/mol), change in enthalpy ΔH° (kJ/mol) and
change in entropy ΔS° (J.K-1mol-1) for cyanide adsorption were calculated from the distribution constant K [10]
by using the following relations:
K
Freundlich
2
Activated
226,7673 0,029620 0,99781 11,98697 1,628459 0,99330
Carbon
x/m, mg/g
Log x / m = logKF + 1 / nlogCe
Table 6. Parameters of Lagmuir and Freundlich for adsorption
of cyanide.
50
0
0
10
20
30
40
50
60
70
80
Ce, mg/g
Figure 8. Langmuir and Freundlich models adjustment at 298
K.
the feasibility and spontaneity of the adsorption process,
where higher negative value reflects a more energetically
favorable adsorption process. The process of adsorption
Cu-AC is more favorable.
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L. Giraldo et al. / Natural Science 2 (2010) 1066-1072
4. CONCLUSIONS
Keeping the adsorptive nature of Cu-AC and Cu-A in
view it is felt desirable to select batch adsorption process
for removal of Cyanide from the industrial wastewater
using activated carbon and γ-alumina. The main advantages of the procedure are:
1) The cost of starting materials for obtaining the catalyst is low and easily available in country.
2) Ease and simplicity of preparation of the catalyst
due to non-corrosive and non-poisonous nature of
activated carbon and alumina.
3) Rapid attainment of phase equilibration and good
enrichment as well fitting of adsorption data with
Langmuir isotherms.
4) The positive value of ΔH° and negative values of
ΔG° indicate the endothermic and spontaneous nature of the adsorption process.
5. ACKNOWLEDGEMENTS
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
The authors wish to thank the Master Agreement established between
Universidad de los Andes and Universidad Nacional de Colombia, and
the Memorandum of Understanding between Departments of Chemistry of both Universities. Additionally, special thanks to Fondo Especial
[16]
de la Facultad de Ciencias and Proyecto Semilla of Universidad de los
Andes for the partial financial of this research.
[17]
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